The parallel axes type transmission has an input shaft provided with a plurality of drive gears; and an output shaft arranged in parallel with the input shaft and provided with a plurality of driven gears respectively engaged (or meshed) with the drive gears. In the transmission, a gear shift operation is carried out by a positive clutch to select a transmission gear train carrying out a power transmission among a plurality of transmission gear trains formed by the drive gears and the driven gears. In a manual transmission, a driver operates a shift lever, whereby the positive clutch is driven and the gear shift operation is carried out. Driving the positive clutch by a hydraulic actuator can achieve an automatic transmission having a base structure on the parallel axes type manual transmission.
Japanese Patent Application Laid-Open No. 60-256656 discloses a parallel axes type automatic transmission having such a structure that a power unit comprising an engine and a transmission is transversely arranged within an engine room (or compartment). This transmission has a drive shaft connected to a crankshaft of the engine, and two hollow input shafts which are concentrically (or coaxially) arranged in an outer side thereof and are respectively provided with a plurality of drive gears. A first friction clutch is arranged in one end portion of the drive shaft to connect one of the input shafts to the drive shaft, and a second friction clutch is arranged in the other end portion of the drive shaft to connect the other input shaft to the drive shaft. This transmission can obtain a predetermined transmission speed by selectively engaging the respective friction clutches and by switching any of the transmission gear trains into a power transmission state.
Further, Japanese Patent Application Laid-Open No. 10-89456 discloses a parallel axes type transmission provided with two friction clutches. This transmission has two input shafts which are concentrically arranged and are respectively provided with a plurality of drive gears. A friction clutch is provided between the respective one end portions of the respective input shafts and a crankshaft. The two friction clutches are switched when switching a positive clutch, and a shift operation is carried out while interlocking with a control of an electronic control throttle valve.
Further, Japanese Patent Application Laid-Open No. 11-264449 discloses a transmission which has an intermediate shaft provided with a drive gear and an output shaft provided with a driven gear, and in which a friction clutch for transmitting a rotation of a crankshaft of an engine to the intermediate shaft. A friction clutch for transmitting the rotation of the crankshaft to the output shaft via an accessory (or intermediate) axis are arranged in an extending (or a longitudinally) direction of the output shaft.
As mentioned above, when it is intended to smoothly carry out the shift operation in the automatic transmission on the basis of the mechanism of the manual transmission while interlocking the friction clutch with the positive clutch, there can be considered that two friction clutches are provided between the engine and the transmission, or in both ends of the transmission, as disclosed in the prior art mentioned above.
However, in the case that the conventional parallel axes type automatic transmission having two friction clutches are applied as the power unit in which the engine and the transmission are transversely arranged within the engine room in a forward (or front) side of a driver seat, the transmission including the friction clutches and devices for controlling an operation of the friction clutches becomes long and large. In particular, when it is intended to employ (or use) a wet type clutch in place of a dry type clutch as the friction clutch in consideration of a controllability, a durability and a reliability, a space in the periphery of the transmission and a freedom of designing the transmission itself are deteriorated, so that it is unavoidable that a whole length of the transmission is increased.
As disclosed in the respective publications mentioned above, in the transmission in which two friction clutches are provided between the engine and the transmission, or two friction clutches are arranged on an extended position of the input shaft, it is limited by the vehicle size in width to have the engine to increase a number of cylinders and the displacement, since the whole length of the transmission is increased.
Further, when the size of the transmission in the width direction becomes large, it is impossible to secure a sufficient space for a crush stroke within the engine room for a head-on or side collision, so that there is an inconvenience that it is hard to satisfy a mountability of the transmission and that an outer appearance and a packaging of the vehicle are largely affected.
Further, in the case of combining a diesel engine with the conventional transmission, it is necessary to assemble a mechanical damper in the transmission for damping a vibration of the engine, and the whole length of the transmission including the damper is increased. In particular, in the case that a torque converter is provided in the crankshaft of the engine, the whole length of the transmission including the torque converter is increased.
Since the friction clutches are arranged on the extension of the input shaft or the output shaft in the prior art as mentioned above, it is difficult to commonly use the parts of the manual transmission and production equipment (or facilities) to the full. Further, it is necessary to achieve a high functional control to increase or reduce an oil pressure for a delicate control of a starting clutch in order for smooth start of the vehicle or for carrying out a smooth gear shift without a shift shock. And the common use between the manual transmission and the automatic transmission must be abandoned (or sacrificed) in order to secure a high productivity.